1. Technical Field
The present invention relates to a position measuring arrangement for determining the position of a spatial point in relation to a reference system.
2. Background Information
Various known approaches exist for determining the position of a spatial point in relation to a reference system.
For one, in the field of mechanical engineering, for example, so-called indirect measuring methods are known, in which the position of a movable spatial point in relation to a stationary reference system is determined by combining position-measuring values from several linear encoders. This can relate to the determination of the position of a movable machine part, such as the so-called “tool center point (TCP)”, for example, in relation to a stationary machine part. Here, linear encoders are employed for detecting the displacement movements of axes which are perpendicularly oriented in relation to each other. The position of the spatial point of interest, such as the TCP, for example, can then be determined in a known manner by combining the positional values for the different axes.
Moreover, so-called direct measuring methods or measuring devices, have become known, in which a direct determination of the position of a movable spatial point, for example, in relation to a stationary reference system takes place. An example of a direct measuring method is described in EP 417 665 A2. The position measuring arrangement for determining the two-dimensional-position and/or orientation of a spatial point, disclosed in this publication, provides several retroreflectors, arranged at fixed distances in a stationary manner in space along a straight line. Furthermore, a scanning unit of the spatial point which will be of interest is arranged, which has a movably mounted scanning mirror, as well as an opto-electronic unit. Periodic grid-like scanning of the space takes place by a lightbeam. A signal can be generated with the aid of the opto-electronic detector unit if, at the time of optical contact between the scanning unit and the respective retroreflector, a back reflection of the lightbeam from the retroreflector in the direction of the scanning unit results. At the time of optical contact between the scanning unit and the retroreflector, the impact angle of a lightbeam reflected back by the retroreflector in the direction of the scanning unit is determined by an evaluation unit which responds to the signal from the opto-electronic detector unit. It is then possible to make a direct determination of the position of the spatial point from the calculations of the angular measurements to several retro-reflectors and with the aid of the known distances between the retroreflectors. In this known arrangement, it is provided to scan the space containing the retroreflectors by a bundled lightbeam. No further details regarding the embodiment of a suitable optical device or the scanning unit can be found in this publication. It should be mentioned that a disadvantage of this method is that it is merely based on a two-dimensional position determination wherein the retroreflectors must be arranged along a straight line. In case of tilting of the straight line in relation to the scanning unit, no optical contact between the retroreflectors and the scanning unit exists anymore, so that false measurements result. Moreover, based on the use of a bundled lightbeam, only a brief optical contact results, as well as only a narrow permissible twist angle between the retroreflectors and the scanning unit, which can also cause errors in the measurement.